1. Field of the Invention
The present invention relates to a switching power supply and, more particularly, to a soft start technique for gradually increasing output voltage of the switching power supply.
2. Description of the Related Art
A boosting-type switching power supply for generating a voltage higher than an input voltage is widely used in various electronic devices. The boosting-type switching power supply has a switching element and an inductor or a transformer. By turning on/off the switching element in discrete time, back electromotive force is generated in the inductor or the transformer to increase the input voltage, and the increased input voltage is output.
In such a switching power supply, to eliminate high frequency components in the boosted voltage and thereby smoothing the boosted voltage, an output capacitor is provided. At the time of starting the switching power supply, the output voltage has to be increased from 0V to a predetermined boosted voltage. In the case of abruptly increasing the output voltage, inrush current may be generated or output voltage may become too high due to overshooting or ringing of the output voltage and thereby influencing the reliability of elements constructing the circuit.
Therefore, conventionally, to prevent generation of the inrush current, a method of providing a control circuit of the switching power supply with a soft start circuit and gradually increasing the output voltage is used (Patent Document 1).    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-216249
However, with the conventional soft start means described in Patent Document 1, the output voltage is increased at predetermined gradient. There has been a problem when a load requiring a very high voltage of about hundreds V is connected to an output terminal of the switching power supply, time required to reach a desired voltage is long.